1. Field of Invention
This invention relates to a process of producing a synthesis gas which has a low inert gas content and is intended for the synthesis of alcohols, particularly of methanol, and of hydrocarbons, and which is produced from coal or heavy hydrocarbons by a gasification under pressure with oxygen and steam followed by a cooling of the raw gas, removal of the impurities, and separation and cracking of methane with oxygen and steam.
2. Discussion of Prior Art
It is known to gasify coal with oxygen and steam under pressure and at elevated temperatures. In such processes the raw gas, which consists substantially of H.sub.2, CO, CO.sub.2 and CH.sub.4, is cooled and is scrubbed with methanol in order to remove impurities, particularly sulfur compounds, such as H.sub.2 S and COS, and CO.sub.2. Because the capacity of the methanol to absorb the impurities strongly increases as the temperature is lowered, the scrubbing with methanol is effected at very low temperatures. The resulting pure gas can be treated, e.g., by the Fischer-Tropsch synthesis, to produce hydrocarbons, alcohols and motor fuels. The sulfur compounds which are recovered by the regeneration of the methanol can be reacted to produce elementary sulfur, e.g., by the Claus process (Linde, Berichte aus Technik und Wissenschaft, 45 (1979), pages 9 to 17). Alternatively, the scrubbing with methanol may be combined with a scrubbing with liquid N.sub.2 to remove CO, CH.sub.4 and Ar and to produce ammonia (Linde I, Berichte zur Technik und Wissenschaft, 45 (1979), pages 3 to 8).
A disadvantage of those known processes resides in that methane constitutes an inert gas, which affects the synthesis. For instance, a higher pressure is required for the reaction of the reactants because the methane acts as a diluent. This reduces the efficiency of the synthesis. On the other hand, residual gases containing methane and the unreacted constituents of the synthesis gases become available at a high rate. For these reasons it is necessary for an improvement of the efficiency of the synthesis, e.g., in the process described in Linde, Berichte aus Technik und Wissenschaft, 45 (1979), pages 9 et seq., to treat said residual gas in an expensive process for a recovery of residual synthesis gas.
It is an object of the invention to avoid these disadvantages and to provide a pure gas which contains H.sub.2 and CO and has a very low inert gas content and can be used for a synthesis of alcohols, particularly of methanol, and of hydrocarbons. It is an object of the invention to provide a simple and economical process for production of such a pure gas.